Balanced delay and resolution for high density servo systems

ABSTRACT

A tape drive-implemented method, according to one embodiment, includes: determining a length of a window of a servo pattern to use for calculating a lateral position estimate, and determining a number of the windows of the servo pattern to use for calculating a lateral position value. A lateral position estimate is calculated for each of the number of the windows of the servo pattern using signals which correspond to each of the number of the windows. Moreover, the lateral position value is calculated by using the lateral position estimates. The lateral position value is used to control a tape head actuator. Other systems, methods, and computer program products are described in additional embodiments.

BACKGROUND

The present invention relates to tape storage systems, and morespecifically, to using high density servo information to determine theposition of a read/write head relative to a magnetic medium.

Timing-based servo (TBS) is a technology which was developed for lineartape drives in the late 1990s. In TBS systems, recorded servo patternsinclude transitions with two different azimuthal slopes, thereby forminga chevron-type pattern. These patterned transitions allow for anestimate of the head lateral position to be determined by evaluating therelative timing of pulses generated by a servo reader reading thepatterns as they are passed over the servo reader.

In a TBS format, the servo pattern is prerecorded in several bandsdistributed across the tape. Typically, five or nine servo pattern bandsare included on a given tape which run about parallel to a longitudinalaxis of the tape. Data is recorded in the regions of tape locatedbetween pairs of the servo bands. In read/write heads of IBM lineartape-open (LTO) and Enterprise tape drives, two servo readers arenormally available per head module, from which longitudinal position(LPOS) information as well as a position error signal (PES) may bederived. Effective detection of the TBS patterns is achieved by asynchronous servo channel employing a matched-filterinterpolator/correlator, which ensures desirable filtering of the servoreader signal.

With the increase in track density that is envisioned for future tapemedia and tape drives, accurately controlling the lateral position ofthe head and/or skew of the head with respect to tape by using feedbackgenerated by reading the TBS patterns becomes increasingly difficult.Conventional servo based implementations may not be sufficientlyaccurate to ensure adequate positioning of the data readers and writersthat move along data tracks. Furthermore, the repetition rate of thehead lateral position estimates may be too low to ensure propertrack-following operation as tape velocity varies during use. Therepetition rate of the head lateral position estimates may additionallybe unable to support future actuators with larger bandwidths. Therefore,it is desirable to achieve head lateral position estimates at both arepetition rate and an accuracy that ensures proper track-followingoperation even at low tape velocities and with large-bandwidthactuators.

Some magnetic tapes may further be augmented with additional featuresthat provide additional functionality. Accordingly, high density (HD)servo patterns may be implemented in place of, or in addition to, thestandard TBS servo patterns.

SUMMARY

A tape drive-implemented method, according to one embodiment, includes:determining a length of a window of a servo pattern to use forcalculating a lateral position estimate, and determining a number of thewindows of the servo pattern to use for calculating a lateral positionvalue. A lateral position estimate is calculated for each of the numberof the windows of the servo pattern using signals which correspond toeach of the number of the windows. Moreover, the lateral position valueis calculated by using the lateral position estimates. The lateralposition value is used to control a tape head actuator.

A computer program product, according to another embodiment, includes acomputer readable storage medium having program instructions embodiedtherewith. The computer readable storage medium is not a transitorysignal per se. Moreover, the program instructions executable by aprocessor to cause the processor to: determine, by the processor, alength of a window of a servo pattern to use for calculating a lateralposition estimate; and determine, by the processor, a number of thewindows of the servo pattern to use for calculating a lateral positionvalue. A lateral position estimate is calculated, by the processor, foreach of the number of the windows of the servo pattern using signalswhich correspond to each of the number of the windows. Moreover, thelateral position value is calculated, by the processor, by using thelateral position estimates. The lateral position value is used, by theprocessor, to control a tape head actuator.

A tape drive, according to yet another embodiment, includes: acontroller comprising logic integrated with and/or executable by thecontroller to cause the controller to: determine a length of a window ofa servo pattern to use for calculating a lateral position estimate, anddetermine a number of the windows of the servo pattern to use forcalculating a lateral position value. A lateral position estimate iscalculated for each of the number of the windows of the servo patternusing signals which correspond to each of the number of the windows.Moreover, the lateral position value is calculated by using the lateralposition estimates. The lateral position value is further used tocontrol a tape head actuator.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network storage system, according to oneembodiment.

FIG. 2 illustrates a simplified tape drive of a tape-based data storagesystem, according to one embodiment.

FIG. 3 illustrates a tape layout, according to one embodiment.

FIG. 4A is a hybrid servo pattern written in a dedicated area of a tapemedium, according to one embodiment.

FIG. 4B is a partial detailed view of a TBS pattern, according to oneembodiment.

FIG. 5A is a HD pattern, according to one embodiment.

FIG. 5B is a graph plotting readback energy vs. frequency for the readerin FIG. 5A.

FIG. 5C is a HD pattern, according to one embodiment.

FIG. 5D is a graph plotting readback energy vs. frequency for the readerin FIG. 5C.

FIG. 6 is a block diagram of a detector for HD patterns, according toone embodiment.

FIG. 7 is a partial block diagram of a servo channel for TBS patterns,according to one embodiment.

FIG. 8 is a block diagram of a detector for HD patterns, according toone embodiment.

FIG. 9 is a block diagram of a track-following servo control loop,according to one embodiment.

FIG. 10 is a graph plotting the power spectral density vs. frequency fortwo different tape speeds.

FIG. 11 is a flowchart of a method, according to one embodiment.

FIG. 12 is a representative view of a lookup table, according to oneembodiment.

FIG. 13 is a representative view of a lookup table, according to oneembodiment.

FIG. 14 is a flowchart of a method, according to one embodiment.

FIG. 15 is a flowchart of a method, according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofmagnetic storage systems, as well as operation and/or component partsthereof for improved position estimation performance using windows of HDservo pattern information from magnetic tape. Accordingly, some of theembodiments described herein successfully improve the accuracy by whichthe position of a tape head may be estimated compared to what has beenpreviously achievable. By combining a number of servo estimates readfrom one or more HD servo tracks by one or more servo readers, a moreaccurate position estimation of the tape head may be achieved. However,a trade-off between accuracy (resolution) of the position, delay, andspatial frequency resolution may exist in the various embodimentsdescribed herein, as will be described in further detail below.

In one general embodiment, a tape drive-implemented method includes:determining a length of a window of a servo pattern to use forcalculating a lateral position estimate, determining a number of thewindows of the servo pattern to use for calculating a lateral positionvalue, receiving signals corresponding to each of the number of thewindows of the servo pattern from a single servo channel, calculating alateral position estimate for each of the number of the windows of theservo pattern, calculating the lateral position value by using thelateral position estimates, and using the lateral position value tocontrol a tape head actuator.

In another general embodiment, a computer program product includes acomputer readable storage medium having program instructions embodiedtherewith, the program instructions executable by a processor to causethe processor to: determine, by the processor, a length of a window of aservo pattern to use for calculating a lateral position estimate,determine, by the processor, a number of the windows of the servopattern to use for calculating a lateral position value, receive, by theprocessor, signals corresponding to each of the number of the windows ofthe servo pattern from a single servo channel, calculate, by theprocessor, a lateral position estimate for each of the number of thewindows of the servo pattern, calculate, by the processor, the lateralposition value by using the lateral position estimates, and use, by theprocessor, the lateral position value to control a tape head actuator.

In another general embodiment, a tape drive includes: a controllercomprising logic integrated with and/or executable by the controller tocause the controller to: determine a length of a window of a servopattern to use for calculating a lateral position estimate, determine anumber of the windows of the servo pattern to use for calculating alateral position value, receive signals corresponding to each of thenumber of the windows of the servo pattern from a single servo channel,calculate a lateral position estimate for each of the number of thewindows of the servo pattern, calculate the lateral position value byusing the lateral position estimates, and use the lateral position valueto control a tape head actuator.

Referring now to FIG. 1, a schematic of a network storage system 10 isshown according to one embodiment. This network storage system 10 isonly one example of a suitable storage system and is not intended tosuggest any limitation as to the scope of use or functionality ofembodiments of the invention described herein. Regardless, networkstorage system 10 is capable of being implemented and/or performing anyof the functionality set forth herein.

In the network storage system 10, there is a computer system/server 12,which is operational with numerous other general purpose or specialpurpose computing system environments or configurations. Examples ofwell-known computing systems, environments, and/or configurations thatmay be suitable for use with computer system/server 12 include, but arenot limited to, personal computer systems, server computer systems, thinclients, thick clients, handheld or laptop devices, multiprocessorsystems, microprocessor-based systems, set top boxes, programmableconsumer electronics, network PCs, minicomputer systems, mainframecomputer systems, and distributed cloud computing environments thatinclude any of the above systems or devices, and the like.

Computer system/server 12 may be described in the general context ofcomputer system-executable instructions, such as program modules, beingexecuted by a computer system. Generally, program modules may includeroutines, programs, objects, components, logic, data structures, and soon that perform particular tasks or implement particular abstract datatypes. Computer system/server 12 may be practiced in distributed cloudcomputing environments where tasks are performed by remote processingdevices that are linked through a communications network. In adistributed cloud computing environment, program modules may be locatedin both local and remote computer system storage media including memorystorage devices.

As shown in FIG. 1, computer system/server 12 in the network storagesystem 10 is shown in the form of a general-purpose computing device.The components of computer system/server 12 may include, but are notlimited to, one or more processors or processing units 16, a systemmemory 28, and a bus 18 that couples various system components includingsystem memory 28 which is coupled to processor 16.

Bus 18 represents one or more of any of several types of bus structures,including a memory bus or memory controller, a peripheral bus, anaccelerated graphics port, a processor or local bus using any of avariety of bus architectures, etc. By way of example, which is in no wayintended to limit the invention, such architectures include IndustryStandard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus,Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA)local bus, and Peripheral Component Interconnects (PCI) bus.

Computer system/server 12 typically includes a variety of computersystem readable media. Such media may be any available media that isaccessible by computer system/server 12, and may include both volatileand non-volatile media, removable and non-removable media.

System memory 28 may include computer system readable media in the formof volatile memory, such as random access memory (RAM) 30 and/or cachememory 32. Computer system/server 12 may further include otherremovable/non-removable, volatile/non-volatile computer system storagemedia. By way of example only, storage system 34 may be provided forreading from and writing to a non-removable, non-volatile magneticmedia—not shown and typically called a “hard disk,” which may beoperated in a hard disk drive (HDD). Although not shown, a magnetic diskdrive for reading from and writing to a removable, non-volatile magneticdisk (e.g., a “floppy disk”), and an optical disk drive for reading fromor writing to a removable, non-volatile optical disk such as a compactdisc read-only memory (CD-ROM), digital versatile disc-read only memory(DVD-ROM) or other optical media may be provided. In such instances,each disk drive may be connected to bus 18 by one or more data mediainterfaces. As will be further depicted and described below, memory 28may include at least one program product having a set (e.g., at leastone) of program modules that are configured to carry out the functionsof embodiments described herein.

Program/utility 40, having a set (at least one) of program modules 42,may be stored in memory 28 by way of example, and not limitation, aswell as an operating system, one or more application programs, otherprogram modules, program data, etc. Each of the operating system, one ormore application programs, other program modules, and program data orsome combination thereof, may include an implementation of a networkingenvironment. It should also be noted that program modules 42 may be usedto perform the functions and/or methodologies of embodiments of theinvention as described herein.

Computer system/server 12 may also communicate with one or more externaldevices 14 such as a keyboard, a pointing device, a display 24, etc.;one or more devices that enable a user to interact with computersystem/server 12; and/or any devices (e.g., network card, modem, etc.)that enable computer system/server 12 to communicate with one or moreother computing devices. Such communication may occur via Input/Output(I/O) interfaces 22. Still yet, computer system/server 12 maycommunicate with one or more networks such as a local area network(LAN), a general wide area network (WAN), and/or a public network (e.g.,the Internet) via network adapter 20. As depicted, network adapter 20communicates with the other components of computer system/server 12 viabus 18. It should be understood that although not shown, other hardwareand/or software components could be used in conjunction with computersystem/server 12. Examples, include, but are not limited to: microcode,device drivers, redundant processing units, external disk drive arrays,redundant array of independent disks (RAID) systems, tape drives, dataarchival storage systems, etc.

Looking to FIG. 2, a tape supply cartridge 120 and a take-up reel 121are provided to support a tape 122. One or more of the reels may formpart of a removable cartridge and are not necessarily part of the tapedrive 100. A tape drive, e.g., such as that illustrated in FIG. 2, mayfurther include drive motor(s) to drive the tape supply cartridge 120and the take-up reel 121 to move the tape 122 over a tape head 126 ofany type. Such head may include an array of readers, writers, or both.

Guides 125 guide the tape 122 across the tape head 126. Such tape head126 is in turn coupled to a controller 128 via a cable 130. Thecontroller 128, may be or include a processor and/or any logic forcontrolling any subsystem of the drive 100. For example, the controller128 may control head functions such as servo following, data writing,data reading, etc. The controller 128 may include at least one servochannel and at least one data channel, each of which include data flowprocessing logic configured to process and/or store information to bewritten to and/or read from the tape 122. The controller 128 may operateunder logic known in the art, as well as any logic disclosed herein, andthus may be considered as a processor for any of the descriptions oftape drives included herein according to various embodiments. Thecontroller 128 may be coupled to a memory 136 of any known type, whichmay store instructions executable by the controller 128. Moreover, thecontroller 128 may be configured and/or programmable to perform orcontrol some or all of the methodology presented herein. Thus, thecontroller 128 may be considered to be configured to perform variousoperations by way of logic programmed into one or more chips, modules,and/or blocks; software, firmware, and/or other instructions beingavailable to one or more processors; etc., and combinations thereof.

The cable 130 may include read/write circuits to transmit data to thehead 126 to be recorded on the tape 122 and to receive data read by thehead 126 from the tape 122. An actuator 132 controls position of thehead 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tapedrive 100 and a host (internal or external) to send and receive the dataand for controlling the operation of the tape drive 100 andcommunicating the status of the tape drive 100 to the host, all as willbe understood by those of skill in the art.

Referring momentarily to FIG. 3, an illustrative tape layout is depictedin accordance with one embodiment. As shown, tape 300 has a tape layoutwhich implements five servo bands Servo Band 0-Servo Band 4, and fourdata bands Data Band 0-Data Band 3, as specified in the LTO format andIBM Enterprise format. The height H of each of the servo bands ismeasured in the cross-track direction 304 which is about orthogonal tothe length L of the tape 300. According to an example, the height H ofeach of the servo bands may be about 186 microns according to the LTOformat. Moreover, a pitch β between the servo bands as shown may beabout 2859 microns, again according to the LTO format.

An exemplary tape head 302 is also shown as having two modules and asbeing positioned over a portion of the tape 300 according to oneapproach. Read and/or write transducers may be positioned on eithermodule of the tape head 302 according to any of the approaches describedherein, and may be used to read data from and/or write data to the databands. Furthermore, tape head 302 may include servo readers which may beused to read the servo patterns in the servo bands according to any ofthe approaches described herein. It should also be noted that thedimensions of the various components included in FIG. 3 are presented byway of example only and are in no way intended to be limiting.

Some tape drives are configured to operate at low tape velocities and/orwith nanometer head position requirements. These tape drives may useservo formats that target Barium Ferrite (BaFe) tape media, 4 or 8 databands, 32 or 64 data channel operation, allow very low velocityoperation, support large-bandwidth actuator operation, and improveparameter estimation to minimize standard deviation of the positionerror signal (PES), thus enabling track-density scaling for tapecartridge capacities up to 100 TB and beyond.

However, according to some embodiments, magnetic tape may further beaugmented with additional features that provide additionalfunctionality. Accordingly, HD servo patterns may be implemented inplace of the standard TBS servo patterns, e.g., as seen in FIG. 3. TheHD servo patterns may be used to improve track-following performance.

In still further embodiments, a standard TBS servo pattern (e.g., asshown in FIG. 3) may be implemented in combination with one or more HDservo patterns (e.g., see FIG. 4A below). One implementation includes ahybrid servo pattern scheme, in which a standard TBS pattern is retainedand additional HD patterns are provided in a dedicated, preferablycurrently unused area of the tape media. This type of pattern may beimplemented by increasing the number of data channels from 16 to 32, andreducing the width of the TBS pattern from 186 microns to 93 microns, insome approaches.

A hybrid servo pattern 410, which includes a standard TBS pattern 402written in a servo track, as well as an HD pattern 404 that is writtenin a track (e.g., dedicated area) of the tape medium 408 is shown inFIG. 4A. In some approaches, significant features of the original TBSpattern 402 are retained, such as a servo frame structure consisting offour servo bursts containing a number of servo stripes, where the servostripes of adjacent servo bursts are written with alternating azimuthalangle. Other parameters of legacy servo patterns, such as the servopattern height and other geometric dimensions, as well as the number ofservo stripes per burst, may be modified as desired.

The HD pattern 404 may include periodic waveforms of various frequenciesalternately written in the length direction L along a longitudinal axisof the tape. The standard TBS pattern 402 may be used to provide initialidentification of the servo band (e.g., by providing a servo band ID);initial positioning of the head 406 on an appropriate servo location;acquisition of initial servo channel parameters, such as tape velocity,lateral head position, head-to-tape skew, LPOS information, etc.; etc.Moreover, the HD pattern 404 may enable more accurate and more frequentestimates of servo channel parameters, thereby achieving improved headpositioning at a much wider range of tape velocities and support forlarger bandwidth head actuation. As such, track-density scaling may beenabled for very large cartridge capacities, as well as improved datarate scaling with host computer requirements through the support of awider velocity range.

The detection of the periodic waveforms forming a HD pattern may beobtained by a detector that implements a complex algorithmic conversion,e.g., such as a Discrete Fourier Transform (DFT), a Fast FourierTransform (FFT), etc. However, this implementation complexity may reducethe flexibility in trade-offs between the rate of generation of servoreader lateral position estimates and the standard deviation of theestimation error. Accordingly, components (e.g., controllers) with highthroughput may desirably be used to process signals derived from a HDpattern in order to reduce the processing time thereof.

In one embodiment, a detector capable of reading a hybrid of TBS and HDpatterns may be implemented. The hybrid detector may be configured toobtain estimates of the energy of relevant spectral frequency componentsin a readback signal from the HD pattern, while also calculatingestimates of the lateral position of the head based on these energies,without applying a DFT or a FFT.

The tape layout 400 of FIG. 4A includes a hybrid servo pattern 410according to one embodiment, in the hybrid servo pattern 410, an HDpattern 404 is written in a space adjacent to a standard TBS pattern402. Three servo readers Servo Reader are assigned to each hybrid servopattern 410 (e.g., servo band). According to the present embodiment,quadrature sequences are not included due to the use of the TBS pattern402, which is converse to products implementing servo functionality inhard-disk drives.

Looking momentarily to FIG. 4B, a partial detailed view of a TBS pattern402 (e.g., a TBS frame) is illustrated according to an exemplaryembodiment. As shown, a plurality of servo stripes 412 together form aservo burst 414, while corresponding pairs of servo bursts 414 formservo sub-frames. In the present embodiment, the servo bursts 414included in the left servo sub-frame each have five servo stripes 412,while the servo bursts 414 included in the right servo sub-frame eachhave four servo stripes 412. The servo stripes 412 included in a givenservo burst 414 are oriented such that they have a same azimuthal sloperepresented by angle α. Moreover, corresponding pairs of servo bursts414 have opposing azimuthal slopes, thereby forming a chevron-typepattern. The height H and thickness t of the servo stripes 412 may varydepending on the servo writer used to write the TBS pattern 402.According to an exemplary approach, which is in no way intended to limitthe invention, the height H may be about 186 μm, and the angle α may beabout 6°, while the thickness t is about 2.1 μm. Moreover, the spacing Sbetween each of the servo stripes 412 and/or the sub-frame length SFLbetween servo bursts 414 having the same azimuthal slope may varydepending on the desired embodiment. According to an exemplary approach,which is in no way intended to limit the invention, the spacing S may beabout 5 μm, while the sub-frame length SFL is about 100 μm. As describedabove, patterned transitions such as that shown in FIG. 4B allow for anestimate of the head lateral position to be determined by evaluating therelative timing of pulses generated by a servo reader reading the servostripes 412 of the servo burst 414 as they are passed over the servoreader.

Referring again to FIG. 4A, the HD pattern 404 of FIG. 4A may includeperiodic waveforms written on adjacent tracks. For example, two periodicwaveforms, characterized by two different spatial frequencies:low-frequency f₁ and high-frequency f₂, where f₂>f₁. However, a widerrange of lateral head displacement is desired. Accordingly, a differentconfiguration of the HD patterns may be used to avoid ambiguity indetermining the lateral displacement.

An HD servo pattern preferably includes periodic waveforms of differingfrequencies alternately written in the lateral (cross-track) direction.Accordingly, HD servo patterns may be able to desirably provide moreaccurate and/or more frequent estimates of servo channel parametersaccording to various embodiments described herein. Looking to FIGS.5A-5D, an HD pattern 500 is shown that overcomes the limited range oflateral head displacement associated with an HD pattern having only twoperiodic waveforms, characterized by two different spatial frequencies.As shown in FIGS. 5A and 5C, at least three frequencies are used for theHD pattern 500 in adjacent tracks, which repeat periodically across theband where the HD pattern is written. In the embodiment of FIGS. 5A and5C, the servo reader (denoted by the block labelled ‘R’) spans wider inthe cross-track direction 502 than a single track, such that at leasttwo tones are detected under any reading conditions at a given time whenthe servo reader R is positioned over the HD pattern 500. Lookingspecifically to FIG. 5A, the reader R spans across both the bottomportion 508 and middle portion 506 of the HD pattern 500. FIG. 5Cillustrates an alternative position for the servo reader R, where thereader R spans across the upper portion 504 and middle portion 506 ofthe HD pattern 500.

The three portions 508, 506, 504 of the periodic waveforms arecharacterized by three different frequencies f₁, f₂, and f₃,respectively, where f₃>f₂>f₁. According to various approaches, eachwaveform may be characterized as having a number of periods in a rangefrom about 25 to about 200, such as 30 periods, 50 periods, 75 periods,100 periods, etc., within a predetermined spacing. More preferably, thepredetermined spacing may be in a range from about 50 μm to about 150μm, such as about 60 μm, about 75 μm, about 100 μm, etc., depending onthe approach. Moreover, the symbol length may be in a range from about0.5 μm to about 3.0 μm, e.g., such as about 1.0 μm, about 1.5 μm, about2.0 μm, etc.

Hence, with continued reference to FIGS. 5A-5D, an edge of one of theportions of the HD pattern 500 may be distinguished from the edge ofanother of the portions. Looking specifically to FIG. 5A, an edge of themiddle portion 506 may be distinguished from an edge of the bottomportion 508 by evaluating the signals read by the servo reader R, whichoverlaps both portions 506, 508. The graph 510 in FIG. 5B identifies thevarious frequencies in the readback signal from servo reader R and theenergy level corresponding to each of the respective frequencies for theposition of the servo reader R shown in FIG. 5A. Energy values may bedetermined in some approaches by integrating over a given amount of time(or distance along the tape). As shown in graph 510, in addition to themiddle frequency f₂, the bottom frequency f₁ is present in the readbacksignal of the servo reader R and may thereby be detected by a spectralanalysis. Furthermore, the energy values of the spectral components f₁and f₂ represent the relation of the servo reader R overlapping themiddle and bottom portions 506, 508. Given that the energy value of thespectral component of frequency f₁ is smaller than the energy value ofthe spectral component of the second frequency f₂, it follows that theservo reader R can be determined to be overlapped with the middleportion 506 more than it is overlapped with the bottom portion 508.Moreover, a comparison of the corresponding energies may be used todetermine a fine position of the servo reader R with respect to amagnetic tape.

Similarly, the graph 520 in FIG. 5D identifies the frequencies in thereadback signal from servo reader R positioned as shown in FIG. 5C, aswell as the energy level corresponding to each of the respectivefrequencies. As shown, frequencies f₂, and f₃ are present in thereadback signal of the servo reader R, and may be detected by a spectralanalysis. Again, the energies of the spectral components for frequenciesf₂, and f₃ indicate that the servo reader R is positioned above theupper and middle portions 504, 506. Given that the energy of thespectral component of frequency f₃ is smaller than the energy of thespectral component of frequency f₂, it follows that the servo reader Ris overlapped with the middle portion 506 more than it is overlappedwith the upper portion 504. Moreover, a comparison of the correspondingenergy values may be used to determine a fine position of the servoreader R with respect to a magnetic tape.

Note that the waveform periods of the three frequencies may be integermultiples of a period T, for example T=241.3 nm, which corresponds tothe highest spatial frequency, which is proportional to 1/T, whenspectral estimation by a DFT/FFT-based detector with a minimum number ofspectral bins for given integration interval is adopted.

FIG. 6 shows a block diagram of a DFT/FFT-based detector 600 configuredfor the computation of the PES from an HD servo pattern comprisingperiodic waveforms. The servo signal from the servo reader 602 isinterpolated using a servo signal interpolator 604 with the timinginformation from a synchronous servo channel 606. The interpolatedsignal samples are then processed by either a DFT-based or a FFT-based(DFT/FFT-based) detector 608 that estimates the signal energy values atfrequencies f₁ and f₂. The DFT/FFT-based detector 608 outputs are inputto a PES computation unit 610, which determines a PES estimate by takingthe difference of the signal energy values.

Ideally, the two periodic waveforms, whose energies are estimated by theDFT/FFT-based detector 608, are sinusoidal waveforms at frequencies f₁and f₂. However, a DFT/FFT-based detector 608 when used for HD patternshas an inherent drawback where the number of spectral components, forwhich an estimate of the energy is provided, depends on the integrationinterval for the DFT (or FFT) computation, and may be very large whenthe integration interval extends over several periods of the fundamentalfrequency, as is typically the case when a low-noise estimation processis used.

Referring momentarily to FIG. 7, a servo channel 606 to extract servoinformation from TBS signals is illustrated according to an exemplaryembodiment which is in no way intended to limit the invention. As shown,a servo signal is input to an analog-to-digital converter (ADC) 702 ofthe servo channel 606, which may in turn be provided to aninterpolation/correlation module 704 and/or an acquisition, monitoringand control module 706. Moreover, an output from the timing-basereference 708 is also provided to the interpolation/correlation module704. Acquisition, monitoring and control module 706 may output a lateralposition estimate and/or a tape velocity estimate which may be furtherused, e.g., according to any of the approaches described herein.Furthermore, optimum signal detection module 710 may output LPOS symbolsand/or a reliability estimate as shown.

Servo channel 606 may operate at a specific clock rate at which itsamples the readback signal of a corresponding tape head. However, aprocessor which implements a controller capable of performing any one ormore of the operations described below (e.g., with respect to methods1100, 1400, 1500), may operate at a different clock rate.

As the number of periodic waveform components forming the readbacksignal of an HD pattern is usually limited to two or three for a givenlateral position, it is advantageous to resort to a low-complexityimplementation of the detector, whereby only estimates of the energy ofthe relevant spectral components at two or three frequencies in thereadback signal of an HD pattern are efficiently computed.

Now looking to FIG. 8, a detector 800 for HD patterns is shown accordingto one embodiment. The detector 800 is configured to operate withperiodic waveforms, which correspond to the components of the readbacksignal of an HD pattern, that are characterized by three frequencies atany time, as illustrated for example in FIGS. 5A-5D according to oneembodiment. With continued reference to FIG. 8, the detector 800includes three digital filters 802, 804, 806 with low implementationcomplexity, each digital filter comprising a second-order infiniteimpulse response (IIR) stage followed by a two-tap finite impulseresponse (FIR) stage, for the estimation of the energy of the readbackHD servo signal at a specific frequency according to the Goertzelalgorithm. Other arrangements and components may be used for the threedigital filters 802, 804, 806 as would be understood by one of skill inthe art upon reading the present descriptions. The waveform periods (innm) corresponding to the three frequencies may be assumed to be integermultiples of a fundamental period, T.

For an accurate estimation of the energies of the three periodicwaveform components in a finite integration interval, the frequencies ofthe periodic waveform components preferably match the characteristicfrequencies of the three digital filters 802, 804, 806, denoted byω₀/2π, ω₁/2π, and ω₂/2π, respectively. When a match is not possible, itis preferred that the frequencies are within about 0.001% to 1.0% of thefrequencies set for the three digital filters 802, 804, 806, and morepreferably a difference of less than about 0.1%. This may be achieved byresampling the output sequence of the ADC 808 at appropriate timeinstants, which may be provided by an interpolator 810, with a time baseobtained from the tape velocity and a given interpolation distanceΔx_(HD), as shown in FIG. 8. The frequency f_(s) of the clock 818, isused as an input to the ADC 808, the counter 820, and the digitalcircuitry of the detector 800. Moreover, the frequency f_(s) of theclock 818 may be either a fixed frequency or a variable frequency.

In one embodiment, the interpolator 810 may be a cubic Lagrangeinterpolator to achieve smaller signal distortion than a linearinterpolator. Of course, any suitable interpolator may be used, as wouldbe understood by one of skill in the art. The output signal samples ofthe interpolator 810 are obtained that correspond with HD servo signalsamples taken at points on the tape that are separated by a stepinterpolation distance equal to Δx_(HD), independently of the tapevelocity. Δx_(HD) is preferably selected such that the conditionT/Δx_(HD)=K is satisfied, where K is a positive integer number. The timebase for the generation of the interpolator output samples may beprovided by an interpolation time computation unit 812, which yields thesequence of time instants {t_(n)}, at which the resampling of the ADCoutput sequence takes place. Time instants {t_(n)} may furthermore beprovided to circular buffer 822.

The detector 800 illustrated in FIG. 8 may be configured such that agiven number of samples is computed by the interpolator 810 within aclock interval T_(s)=1/f_(s). However, doing so may set a limit on themaximum tape velocity at which the detector 800 may operate, the maximumtape velocity represented by 2Δx_(HD)/T_(s). The maximum tape velocitysupported by the detector 800 may be increased by allowing a largernumber of samples to be computed by the interpolator 810 within a singleclock interval, but doing so also increases computational complexity.

For a fixed tape velocity, the time instants {t_(n)} may be uniformlyspaced by T_(I) seconds, where T₁ denotes the time interval that ittakes for the tape to travel over a distance equal to the stepinterpolation distance Δx_(HD). The estimation of the time intervalT_(I) is performed by a step interpolation time computation unit 814,which computes T_(I)=Δx_(HD)/v_(est), i.e., the ratio between Δx_(HD)and the estimate of the instantaneous tape velocity v_(est), which maybe obtained from the TBS channel in one approach. The TBS channel mayoperate as a synchronous TBS channel according to one embodiment. Theaverage number of interpolated signal samples generated per ADC clockinterval is given by the ratio T_(s)/T_(I), where T_(s)=1/f_(s) denotesthe clock interval. The ADC clock frequency, f_(s), may be a fixedfrequency in one approach, or a variable frequency in another approach.

In one embodiment, the HD detector 800 may be configured to estimate thetape velocity to determine time instants at which to obtain interpolatedsignal samples to input to the Goertzel algorithm as filtering elementsbased on an output of a TBS channel of the tape drive configured toprocess a TBS pattern written on the servo band of the magnetic tapemedium.

However, tape velocity during use may not be fixed for a given operationand/or between tapes. Moreover, tape drives are expected to operate overa range of tape speeds, e.g. in the range of 1 to 6 m/s, in order tomatch variable host data rates. The combination of periodic waveforms ofvarious frequencies along HD servo patterns and the variable tape speedleads to a large range of update rates (e.g., frequencies) and avariable delay at which the position estimates are provided to the servocontroller. Accordingly, an improved process of adjusting lateralposition estimation resolution with respect to estimation delay, spatialfrequency resolution and estimation resolution (e.g., noise) maydesirably be implemented, as will be described in further detail below.

Referring still to FIG. 8, in another embodiment, the HD detector 800may be configured to compute a head lateral position estimate for coarsepositioning of the servo reader based on an output of a TBS channel ofthe tape drive. Also, the HD detector 800 may be configured to adjustsettings for at least one digital filter according to waveform frequencycomponents of the HD servo signal estimated based on the head lateralposition estimate. For example, the setting ω_(i) of the i-th digitalfilter may be adjusted based on the coarse position estimate and theknown frequency ω_(i)=2πf_(i) of the HD patterns located at thatestimated (coarse) lateral position. In another example, the settings ofthe i-th digital filter may be adjusted based on the coarse positionestimate and the combination of symbol length, integration interval,etc., of the HD patterns located at that estimated (coarse) lateralposition.

The HD detector 800 receives, as inputs, values of the threecharacteristic frequencies {ω₀, ω₁, ω₂}, with ω_(i)=2πf_(i) from whichthe coefficients of the digital filters 802, 804, 806 are obtained.These frequencies may be obtained from the knowledge of the servo readerlateral position provided by the TBS channel in one embodiment, asdescribed above. The number (N) is the number of samples over which theestimates of the energies of the periodic waveforms are computed. Ndetermines the length of the integration interval, and therefore, alsodetermines the spatial frequency resolution. Assuming N is even, N/2 isthe number of frequencies for which energy estimates would be providedby a DFT/FFT-based HD detector that operates over N samples. N may beobtained from the tape drive memory in one embodiment.

Typically, N is about 100 or larger. Multiplication of the three energyestimates by gain factors g_(i), for i=0, 1, 2, is provided tocompensate for the different attenuations that the readback HD servosignal may experience at different frequencies, where the normalizationg₁=1 may be assumed. Hence, a lateral position estimate of the HD servoreader 816, and hence a position error signal from the knowledge of thetarget head position, may be obtained by a linear combination of thethree energy estimates. Note that the maximum number of spectralestimates that are computed at any time is determined by the maximumnumber of tracks that may be read by the HD servo reader 816, which mayequal three in some approaches, and not by the overall number of tonesin the HD servo pattern, which may be larger than three. In a case wherethe number of tones is larger than three, the values of the threecharacteristic frequencies {ω₀, ω₁, ω₂} that are provided to the HDdetector 800 may be derived from knowledge of the lateral positionestimate obtained from the TBS channel, as mentioned above.

In another embodiment, the HD detector 800 may be implemented without aninterpolator 810, but with digital filters configurable to adjust theirsettings according to the waveform spatial frequency components of theHD servo signal read from the magnetic tape medium and the tapevelocity. Adjustment of the digital filters settings may be based on acoarse head lateral position estimate and/or a tape velocity estimatecomputed based on an output of a TBS channel of the tape drive.

In an alternate embodiment, an HD detector may implement additionaldigital filters, in excess to the digital filters used to estimate theenergies at the frequencies corresponding to the patterns written on thetracks being read simultaneously by the HD servo reader 816. The one ormore excess digital filters may be used to simplify reconfiguration ofthe detector when the target lateral position changes and, therefore,the input values of frequencies {ω_(x)} vary dynamically.

In a further embodiment, the one or more excess digital filters may beused to distinguish HD patterns characterized by a small number ofspectral components/lines from broadband noise and/or data signals. Thismay be achieved by choosing the characteristic frequency ω_(i) of theexcess digital filter such that it measures a spectral component at afrequency that is not used by the HD patterns.

The outputs |X_(i,t)|² from the three digital filters 802, 804, 806 areprovided to a PES computation unit 824, which provides a position errorestimate (ε_(t)) at given time t.

Other components of the HD detector 800 may operate as would be known toone of skill in the art, and are omitted here for the sake of clarity ofthe described embodiments.

As described above, a signal obtained while reading a HD servo patternmay be used to estimate the lateral position of a tape head relative toa magnetic medium having the HD servo pattern written thereon. UnlikeTBS patterns, HD servo patterns may not have defined, reoccurringframes. Therefore, the readback signal corresponding to a given windowof the HD servo pattern may be used to derive the lateral positionestimate. Again, the energy level corresponding to each of therespective frequencies in a readback signal from a given window of a HDservo pattern read by a servo reader may be used to determine a lateralposition estimate for a tape head relative to the magnetic medium, e.g.,see description of FIGS. 5A-5D above. Moreover, a comparison of thedifferent energy values may be used to determine a fine lateral positionestimation of the tape head with respect to the magnetic medium(magnetic tape).

Once a lateral position estimate has been determined, a track-followingservo control loop may be used to calculate a PES by subtracting thedesired position from the position estimate. Looking now to FIG. 9, anexemplary track-following servo control loop 900 is illustrated inaccordance with one embodiment. As an option, the present servo controlloop 900 may be implemented in conjunction with features from any otherembodiment listed herein, such as those described with reference to theother FIGS. However, such servo control loop 900 and others presentedherein may be used in various applications and/or in permutations whichmay or may not be specifically described in the illustrative embodimentslisted herein. Further, the servo control loop 900 presented herein maybe used in any desired environment. Thus FIG. 9 (and the other FIGS.)may be deemed to include any possible permutation.

The input to the servo control loop 900 is shown as being calculated bysubtracting the desired position y_(ref) from the position estimatey_(pos), thereby forming the PES, as described above. Moreover, the PESis passed to a track-following compensator 902 (e.g., a controller)which in turn produces a signal u_(y) which may be combined withvibration disturbances d_(v) before being passed to a current driver(not shown) and an actuator control module 904. The input control signalto the actuator control module 904 is used to move the correspondingtape head in a desired direction based on the PES, as would beappreciated by one skilled in the art after reading the presentdescription. The signal y at the output of the actuator control module904 indicates the lateral position of the tape head. Moreover, thesignal y may be combined with lateral tape motion (LTM) disturbancesd_(LTM) before being passed to a delay module 906 and a remainder of acontrol system. LTM may be caused by imperfections in the tape transportsystem rollers, reels, motors, etc.

As shown, the output from delay module 906 is redirected back and usedto determine the next PES corresponding to a subsequent servo sub-frameread by a servo reader. In doing so, the servo control loop 900 may beable to keep the tape head at a desired position relative to a magnetictape to enable efficient writing to and/or reading from the magnetictape.

Looking to graph 1000 of FIG. 10, plots for power spectral density ofLTM are shown for two different tape speeds. In other words, the plotsincluded in graph 1000 correspond to LTM rather than any head activationcaused by a controller. Here, the LTM caused by transporting the tapefrom one reel to the other results in a higher amount of mechanicalmotion at lower frequencies than at higher frequencies. Tracking errorsmay be affected by one or more parameters such as position estimationresolution, system delay and/or sampling times, LTM at high and/or lowtape speeds, mechanical coupling, vibration environments, etc. As thefrequency increases, a noise floor dominated by estimation noise istypically reached. Thus, at higher frequencies, control loops may belimited by how accurately the position of the tape head may beestimated.

In order to lower the noise floor and thereby improve the accuracy bywhich the position of the tape head may be estimated compared to whathas been previously achievable, various embodiments described herein maycombine (e.g., average) a number of previous servo estimates (lateralposition estimates) from a single servo reader, or more than one servoreader. By combining more than one servo estimate over time oreffectively some distance on tape, a more accurate determination of theposition of the tape head may be achieved. However, although combininggreater than one servo estimates allows for a more accuratedetermination of the position of the tape head, it also introduces adelay in making such a determination, e.g., dependent on tape speed.Attempts to adjust the size (e.g., length) of the window of the HD servopattern used to determine a lateral position estimate have previouslybeen made in an attempt to reduce the delay experienced. However,adjusting the window length detrimentally affects the spatial frequencyresolution of the system as tape speed constantly changes duringpractice. Thus, a trade-off between accuracy (resolution) of theposition estimate, delay, and spatial frequency resolution may exist inthe various embodiments described herein, as will soon become apparent.

Looking to FIG. 11, a flowchart of a method 1100 is illustrated inaccordance with one embodiment. The method 1100 may be performed inaccordance with the present invention in any of the environmentsdepicted in FIGS. 1-10, among others, in various embodiments. Of course,more or less operations than those specifically described in FIG. 11 maybe included in method 1100, as would be understood by one of skill inthe art upon reading the present descriptions.

Each of the steps of the method 1100 may be performed by any suitablecomponent of the operating environment. For example, in someembodiments, any one or more of the operations included in method 1100may be performed or implemented by a tape drive (e.g., see 100 of FIG.2). In other various embodiments, the method 1100 may be partially orentirely performed by a controller, a processor, etc., or some otherdevice having one or more processors therein. The processor, e.g.,processing circuit(s), chip(s), and/or module(s) implemented in hardwareand/or software, and preferably having at least one hardware componentmay be utilized in any device to perform one or more steps of the method1100. Illustrative processors include, but are not limited to, a centralprocessing unit (CPU), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA), etc., combinationsthereof, or any other suitable computing device known in the art.

As shown in FIG. 11, operation 1102 includes determining a current speedof a magnetic tape, e.g., from a servo track on the magnetic tape. Thecurrent speed of a magnetic tape may be determined by evaluating thereadback signal from a servo reader reading the servo stripes of a TBStrack, as would be appreciated by one skilled in the art after readingthe present description.

Moreover, operation 1104 includes determining a length of a window of aHD servo pattern to use for calculating average energies of HDwaveforms, from which a lateral position estimate is obtained. The HDservo pattern may be written on a magnetic tape, e.g., of a type knownin the art. As previously mentioned, unlike TBS patterns, HD servopatterns may not have defined, reoccurring frames. Therefore, thereadback signal corresponding to a given window of the HD servo patternmay be used to derive (calculate) a lateral position estimate, e.g., bycomparing energy values (levels) corresponding to each of the respectivefrequencies therein (e.g., see FIGS. 5A-5D above).

In some approaches, energy values may be determined by integrating thepower of the readback signal over a given amount of time (or distancealong the tape). It follows that the length (size) of the window of a HDservo pattern may determine the spatial frequency resolution for a givenimplementation. This is because integrating over a longer time frame (orover a larger area) provides a better spectrally resolved result.However, tape velocity during use may not be constant for one or moregiven operations and/or between tapes, e.g., there might be velocityfluctuations around a target velocity. However, the shorter theintegration time, the smaller the effect of velocity variations on theenergy estimation.

In sharp contrast to the foregoing conventional shortcomings, a minimumwindow length (D_(w)) is preferably chosen in various embodiments, suchthat the spatial frequency resolution (˜1/D_(w)) is sufficiently largeto allow reliable energy estimation without increasing the length of thewindow, and sufficiently small to provide robustness against velocityvariations. The minimum window length D_(w) chosen may be based (atleast in part) on a current, intended, anticipated, etc., tape speed(e.g., see lookup table 1300 of FIG. 13 below). According to anexemplary embodiment, the length of the window D_(w) chosen maycorrespond to the minimum anticipated tape speed such that unnecessarydelay is not introduced to the system during operation at low tapespeeds. Moreover, when the tape speed increases above the minimumanticipated tape speed, more than one lateral position estimate derivedfrom more than one window length may be combined, e.g., as described infurther detail below. According to various illustrative approaches,which are in no way intended to limit the invention, the minimum windowlength D_(w) may be in a range from about 5 μm to about 50 μm, but couldbe higher or lower.

With continued reference to FIG. 11, operation 1106 includes determininga number of the windows of the HD servo pattern to use for calculating alateral position value. As mentioned above, one lateral positionestimate may be derived (calculated) from each respective window lengthof the HD servo pattern. Thus, operation 1106 is essentially determininga number of lateral position estimates to use for calculating thelateral position value. In other words, operation 1106 may essentiallydetermine a number of samples for calculating the lateral positionvalue. According to the present description, a “lateral position value”is intended to represent the current (or a most recently determinable)position of a tape head relative to a magnetic tape being passedthereover. Moreover, the lateral position value may be measured in across-track (lateral) direction (e.g., see 304 of FIG. 3).

As alluded to above, using a larger number of lateral position estimatesderived from a larger number of windows to calculate the lateralposition value achieves a more accurate result because doing soessentially increases the combined amount of time (or distance along thetape) which the energy values are being integrated over. However,although using a larger number of lateral position estimates derivedfrom a larger number of windows to calculate the lateral position valueachieves a more accurate result, a corresponding amount of delay is alsointroduced. For example, it takes about twice as long for a servo readerto pass over an amount of tape corresponding to a number of windows ofequal length involved with calculating 4 separate lateral positionestimates than it does to pass over a number the windows involved withcalculating 2 lateral position estimates. Accordingly, there is a longerdelay in achieving the 4 separate lateral position estimates than thereis to achieving the 2 lateral position estimates. As the number oflateral position estimates used to calculate a lateral position valueincrease, so does the delay. Thus, a trade-off between the accuracy(resolution) of the lateral position value, and the delay in determiningthe lateral position value exists. Moreover, this trade-off is alsoaffected by the spatial frequency resolution associated with the chosenlength of the window.

According to various embodiments described herein, this trade-offbetween accuracy and delay may desirably be optimized as a function oftape speed, as tape speed may change multiple times during operation.Changes in tape speed may result from effects such as host systemprocessing bandwidth, the rate at which data is received, availablethroughput, etc. Although delay increases with the number of lateralposition estimates used to calculate a lateral position value, theactual amount of delay depends on the speed at which the tape is beingpassed over the servo readers. For example, at lower tape speeds (e.g.,from about 1 m/s to about 3 m/s), the amount of delay associated withdetermining 3 separate lateral position estimates from 3 separatewindows of a HD servo track is greater than the amount of delayassociated with determining 3 separate lateral position estimates from 3separate windows of a HD servo track on a tape traveling at fasterspeeds, again assuming each of the windows are equal in length. This isbecause faster tape speeds (e.g., from about 4 m/s to about 9 m/s) allowfor the servo reader to read a larger number of windows of the HD servotrack per unit of time than at lower tape speeds, thereby allowing forthe lateral position estimates to be determined more often for fastertape speeds also.

It follows that a greater number of windows, and therefore a greaternumber of lateral position estimates, may be used to calculate a lateralposition value when tape speeds are higher (thereby maintaining theresulting delay within desired limits), while a lower number of windows,and therefore a fewer number of lateral position estimates, may be usedto calculate a lateral position value when tape speeds are lower (as thedelay associated with lateral position estimation is larger). In otherwords, the lateral position estimation delay is larger at lower tapespeeds, e.g., due to fixed window lengths in HD servo patterns, whilelateral position estimation delays are lower at higher tape speeds. Insome approaches, the tape speed may be so low that only one window, andtherefore only one lateral position estimate, may be used forcalculating the lateral position value. In other words, the lateralposition estimate derived from each window of the HD servo pattern maybe used by the track-following servo control loop as the actual lateralposition value. However, using each lateral position estimateindividually (without combining any estimates) may only be implementedtemporarily. The number of windows, and therefore the number of lateralposition estimates, used to calculate the lateral position value may beincreased as the speed of tape increases.

It follows that the number of the windows of the HD servo patterndetermined in operation 1106 to be used for calculating the lateralposition value may depend on (be based on) the speed of tape in someapproaches. Again, the lower amounts of delay corresponding to fastertape speeds allow for more windows of the HD servo pattern to be readand for more lateral position estimates to be gathered and used tocalculate a lateral position value, while the higher amount of delayassociated with lower tape speeds does not. Accordingly, operation 1102may be performed when the current speed of the magnetic tape is desiredin order to determine the number of windows of the HD servo pattern touse. However, in some approaches the number of windows of the HD servopattern may be determined based on other information. For instance, thenumber of windows of the HD servo pattern determined in operation 1106may be based on any one or more of tape speed, vibration conditions,media type, mechanical coupling, environmental conditions, imperfectionsin the tape transport system, etc.

According to an example, which is in no way intended to limit theinvention, the number of windows of the HD servo pattern, and thereforethe number of lateral position estimates, may be determined by using alookup table (e.g., stored in memory). The lookup table may define howmany lateral position estimates from windows of the HD servo patternshould be combined (e.g., averaged) as a function of a tape speed indexs_(i) (velocity of tape), e.g., before being used in a track-followingcontrol loop. Looking momentarily to FIG. 12, an exemplary lookup table1200 is illustrated. As shown, a speed index s_(i) input is applied todetermine the number of lateral position estimates c_(i) to use whencalculating the lateral position value, which directly correlates to thenumber of windows of the HD servo pattern to use for calculating thelateral position value. The number of lateral position estimates maythereby be output and applied when performing operation 1106.

The entries in lookup table 1200 of FIG. 12 may be predetermined,determined using an equation, selected by a user, based on modeling,etc. According to some approaches, the entries in lookup table 1200 ofFIG. 12 may be determined using Equation 1 below. It should also benoted that lookup table 1200 may incorporate additional input parameterswhich may be used to determine the number of lateral position estimatesc_(i) from a corresponding number of windows of the HD servo pattern touse/output therefrom. Additional input parameters may include thoserelated to one or more of: vibration conditions, media types,environmental conditions (e.g., temperature, humidity, etc.), etc.Moreover, the contents of the lookup table 1200 may be static in someapproaches, whereby the outputs are fixed relative to given inputvalues. However, in other approaches the contents of the lookup table1200 and/or their interrelationships may be adaptive.

Referring again to method 1100, operation 1108 includes receivingreadback signals corresponding to each of the number of the windows ofthe HD servo pattern from a single servo channel. Moreover, operation1110 includes calculating a lateral position estimate for each of thenumber of the windows of the HD servo pattern, e.g., using the readbacksignals received in operation 1108. As described above, a lateralposition estimate may be calculated from a window of the HD servopattern by using the readback signal form a servo reader reading the HDservo channel. According to a specific approach, a lateral positionestimate may be calculated from a window of the HD servo pattern byevaluating the energy values of the spectral components in the signalsread from the window by a corresponding servo reader (e.g., see FIGS.5A-5D above). Moreover, a comparison of the corresponding energy valuesmay be used to determine a fine position of the servo reader withrespect to a magnetic tape.

Accordingly, the rate at which the lateral position estimates may becalculated may depend on the rate at which readback signals from each ofthe number of the windows of the HD servo pattern are received, whichmay in turn depend on the speed of tape, an amount of processingbandwidth, the window length, etc., depending on the desired embodiment.In some approaches, the lateral position estimate may also include(e.g., be supplemented by) a tape velocity estimate, tape skew estimate(relative to an orientation of a magnetic head), etc., which may bederived from a servo channel corresponding to a TBS pattern, as would beappreciated by one skilled in the art after reading the presentdescription. Moreover, supplemental values may be calculated using oneor more of the additional estimates received. For instance, a magnetictape velocity value may be calculated using received tape velocityestimates, a magnetic tape skew value (relative to an orientation of themagnetic head) may be calculated using received tape skew estimates,etc., depending on the desired approach. Once calculated, thesupplemental values may be combined with a lateral position value beforebeing used to control a tape head actuator, as will be described infurther detail below (e.g., see operation 1118).

It should be noted that although operation 1108 includes receivingsignals corresponding to each of the number of the windows of a HD servopattern from a single servo channel, signals corresponding to windows ofother HD servo patterns may also be received from other servo channels,e.g., as will be described in further detail below.

Although lateral position estimates may be calculated using the readbacksignal corresponding to a window of the HD servo pattern received from aservo reader reading the HD servo pattern, in some approaches, lateralposition estimates may be retrieved from memory. It follows that lateralposition estimates received from a servo channel may be stored in memorysuch that they may be available for future use. Accordingly, optionaloperation 1112 includes storing at least some of the lateral positionestimates calculated in operation 1110, in a designated location inmemory. The number of previously derived lateral position estimatesstored in memory may depend on the desired approach. In some approaches,the previously derived lateral position estimates may be stored inmemory in a shifting manner, whereby an oldest one or more of thelateral position estimates stored in memory are replaced by a newlyreceived one or more lateral position estimates, e.g., in afirst-in-first-out (FIFO) or ring-buffer. Thus, the number of lateralposition estimates stored in memory at any given time may be fixed.However, in other approaches the number of lateral position estimatesstored in memory at a given time may vary. Moreover, the memory may beof any desired type (e.g., a buffer, a lookup table, history, registers,etc.) and may at least be electrically coupled to the device performingthe operations of method 1100.

Optional operation 1114 also includes retrieving one or more of thestored lateral position estimates from the memory to calculate thelateral position value. Again, although lateral position estimates maybe derived from the readback signal received from a HD servo pattern, insome approaches, lateral position estimates may be retrieved from memoryin order to calculate the lateral position value. Thus, depending on thedesired approach, any number of lateral position estimates previouslydetermined and stored in memory may be retrieved and preferably used tocalculate a new lateral position value.

Method 1100 also includes calculating the lateral position value usingthe number of lateral position estimates. See operation 1116. Dependingon the approach, the number of lateral position estimates may be used tocalculate the lateral position value differently. For instance, in someapproaches the lateral position value may be calculated by averaging thenumber of lateral position estimates, e.g., according to a number ofsamples. However, in other approaches the lateral position value may becalculated by implementing a weighted averaging, e.g., such that ahigher weight is assigned to the most recent sample(s), and a lowerweight is assigned to older samples. In some approaches, the way inwhich the lateral position value is calculated may depend on the numberof lateral position estimates that are available.

Furthermore, operation 1118 includes using the lateral position value tocontrol a tape head actuator. Once a lateral position valuecorresponding to the position of a tape head relative to a magnetic tapehas been determined (e.g., calculated), that value is preferably used toadjust the position of the tape head, e.g., in a conventional matter.According to some approaches, the lateral position value may be sent toa track-following actuator, a current driver for the track-followingactuator, and/or a controller coupled to the track-following actuator.For example, operation 1118 may include implementing the lateralposition value in a track-following servo control loop, e.g., as shownin FIG. 9.

It should be noted that one or more the operations included in method1100 may be repeatedly performed during run-time, e.g., while writingto, reading from, performing a coarse look-up on, etc. a magnetic tape.Moreover, according to different approaches, the operations included inmethod 1100 may be performed synchronously in dependence upon tapespeed, or asynchronously independent of tape speed. According to asynchronously implemented example, the operations included in method1100 may be performed in direct response to passing over another windowof the HD servo pattern, thereby resulting in a new readback signalbeing received and a new lateral position estimate being determined.However, an asynchronously implemented example may include theoperations in method 1100 being performed at a predetermined frequency,such as 20 kHz or any other desired value.

As mentioned above, although method 1100 includes receiving signalscorresponding to each of the number of the windows of a HD servo patternfrom a single servo channel, signals corresponding to windows of otherHD servo patterns may also be received from other servo channels. Insome approaches, lateral position estimates derived from a readbacksignal corresponding to windows of a first HD servo pattern read by afirst servo reader may be combined (e.g., averaged) with lateralposition estimates derived from a readback signal corresponding towindows of a second HD servo pattern read by a second servo reader. Forexample, readback signals corresponding to a number of windows of a HDservo pattern may be received from two servo channels, three servochannels, four servo channels, multiple servo channels, etc., and may becombined (e.g., averaged together) to form a combined lateral positionvalue. By combining lateral position estimates derived from readbacksignals from more than one HD servo track read by more than one servoreader, the accuracy (resolution) of the resulting lateral positionvalue is improved because each of the servo readers may estimate thesame lateral position, but the noise corresponding to the lateralposition estimate made from each of the servo readers is uncorrelated.

Thus, combining multiple lateral position estimates may causecorresponding noise to be reduced, thereby allowing a control loop tooperate at a lower rate, while maintaining an improved PES performance.Moreover, a lateral position value formed by combining lateral positionestimates from two or more parallel servo channels may be buffered insome approaches, e.g., such that it may be provided to a microcontrollerat a reduced interrupt rate.

Looking now to FIG. 14, a flowchart of a method 1400 is illustrated inaccordance with one embodiment. The method 1400 may be performed inaccordance with the present invention in any of the environmentsdepicted in FIGS. 1-10, among others, in various embodiments. Of course,more or less operations than those specifically described in FIG. 14 maybe included in method 1400, as would be understood by one of skill inthe art upon reading the present descriptions.

Each of the steps of the method 1400 may be performed by any suitablecomponent of the operating environment. For example, in someembodiments, any one or more of the operations included in method 1400may be performed or implemented by a tape drive (e.g., see 100 of FIG.2). In other various embodiments, the method 1400 may be partially orentirely performed by a controller, a processor, etc., or some otherdevice having one or more processors therein. The processor, e.g.,processing circuit(s), chip(s), and/or module(s) implemented in hardwareand/or software, and preferably having at least one hardware componentmay be utilized in any device to perform one or more steps of the method1400. Illustrative processors include, but are not limited to, a centralprocessing unit (CPU), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA), etc., combinationsthereof, or any other suitable computing device known in the art.

As shown in FIG. 14, operation 1402 includes determining a first lengthof a window of a HD servo pattern to use for calculating a first lateralposition estimate, while operation 1404 includes determining a secondlength of a window of a HD servo pattern to use for calculating a secondlateral position estimate. The readback signal corresponding to a givenwindow of the HD servo pattern may be used to derive (calculate) alateral position estimate, e.g., by comparing energy levelscorresponding to each of the respective frequencies therein (e.g., seeFIGS. 5A-5D above). Moreover, the length (size) of the window of a HDservo pattern may determine the spatial frequency resolution for a givenimplementation. Thus, a minimum window length (D_(w)) is preferablychosen such that the spatial frequency resolution (˜1/D_(w)) issufficiently large to allow reliable energy estimation withoutincreasing the length of the window, and sufficiently small to providerobustness against velocity variations.

Furthermore, 1406 includes determining a first number of the windows ofthe HD servo pattern to use for calculating a first lateral positionvalue, while operation 1408 includes determining a second number of thewindows of the HD servo pattern to use for calculating a second lateralposition value. As mentioned above, one lateral position estimate may bederived (calculated) from each respective window length of the HD servopattern. Thus, operations 1406 and 1408 are essentially determining anumber of lateral position estimates to use for calculating the firstand second lateral position values respectively.

The number of lateral position estimates used to calculate a givenlateral position value may depend on the tape speed. Faster tape speedsmay allow for a greater number of lateral position estimates to be usedto calculate a respective lateral position value, thereby achieving amore accurate result without introducing an undesirable amount of delayin doing so. However, slower tape speeds may call for a fewer number oflateral position estimates to be used in calculating the respectivelateral position value, thereby sacrificing accuracy in order to avoiddetrimental delay. However, determining the first and/or second numberof the windows of the HD servo pattern to use for calculating therespective lateral position value may vary depending on one or more ofvibration conditions, media type, mechanical coupling, environmentalconditions, imperfections in the tape transport system, etc.

According to some approaches, the second number of windows of the HDservo pattern may be determined according to (based on) the first numberof windows of the HD servo pattern. In other words, the first and secondnumber of windows of the HD servo pattern may be the same in someapproaches. However, the first and second number of the windows of theHD servo pattern may be different, e.g., depending on the desiredembodiment.

According to an example, which is in no way intended to limit theinvention, the first and/or second number of windows of the HD servopattern, and therefore the respective number of lateral positionestimates, may be determined by using a lookup table (e.g., stored inmemory). The lookup table may define how many lateral position estimatesc_(i) from windows of the HD servo pattern should be combined (e.g.,averaged) and/or the length D_(w) of the windows themselves, as afunction of a tape speed index s_(i) (velocity of tape), e.g., beforebeing implemented in a track-following control loop. Looking momentarilyto FIG. 13, an exemplary lookup table 1300 is illustrated.

As shown, a speed index s_(i) input is applied to determine the numberof windows of the HD servo pattern c_(i) (which directly correlate tothe number of lateral position estimates c_(i)) to use when calculatingthe lateral position value. Moreover, the speed index s_(i) input may beapplied to determine the length D_(w) of the windows themselves. Thenumber of lateral position estimates to combine for calculating alateral position value, and/or the length D_(w) of the windows used todetermine such lateral position estimates, may thereby be output andapplied when performing any one or more of operations 1402-1408.

The entries included in a lookup table such as lookup table 1200 of FIG.12 may be determined using any desired approach that would becomeapparent to one skilled in the art upon reading the present description.In some approaches, the entries in lookup table 1200 of FIG. 12 may bedetermined using an equation such as Equation 1 below. Accordingly, thenumber of lateral position estimates c_(i) from windows of a HD servopattern to be combined for forming a lateral position value and windowlengths D_(w) may be chosen for ranges of tape speeds. It should also benoted that lookup table 1200 may incorporate additional input parameterssuch as vibration conditions, media types, environmental conditions(e.g., temperature, humidity, etc.), etc., which may be used todetermine an output therefrom. Moreover, the contents of the lookuptable 1200 may be static in some approaches (e.g., predetermined),whereby the outputs are fixed relative to given input values. However,in other approaches the contents of the lookup table 1200 and/or theirinterrelationships may be adaptive.

Referring again to method 1100, operation 1410 includes receivingreadback signals corresponding to each of the first number of thewindows of the HD servo pattern from a first servo channel. Moreover,operation 1412 includes receiving readback signals corresponding to eachof the second number of the windows of the HD servo pattern from asecond servo channel that is preferably different than the first servochannel. In some approaches, the first and second servo channels maycorrespond to servo tracks read by respective servo readers on oppositesides of a data band on a magnetic tape (e.g., see FIG. 4A). However, inother approaches, the first and second servo channels may correspond toservo tracks on the same side of a data band on a magnetic tape, read bytwo different servo readers.

Lateral position estimates may thereby be calculated for each of thefirst and second number of windows using the readback signals receivedin operations 1410, 1412. See operation 1414. One lateral positionestimate may be calculated from each window of the HD servo pattern byusing the readback signal corresponding thereto. Again, a lateralposition estimate may be calculated from a window of the HD servopattern in some approaches by evaluating the energy values of thespectral components in the signals read from the window by acorresponding servo reader (e.g., see FIGS. 5A-5D above). Moreover, acomparison of the corresponding energy values may be used to determine afine position of the servo reader with respect to a magnetic tape. Insome approaches, the lateral position estimate may also include (e.g.,be supplemented by) a tape velocity estimate, tape skew estimate, etc.

Furthermore, optional operation 1416 includes storing in memory at leastsome of the lateral position estimates calculated in operation 1414,e.g., according to any of the approaches described above. Again,although lateral position estimates may be calculated using the readbacksignal corresponding to a window of the HD servo pattern received from aservo reader reading the HD servo pattern, previously determined lateralposition estimates may be retrieved from memory, e.g., to be used tocalculate one or more corresponding lateral position values. Thus,depending on the desired approach, any number of lateral positionestimates previously determined and stored in memory may be retrievedand preferably used to calculate one or more new lateral positionvalues. Accordingly, optional operation 1418 may include retrieving oneor more of the stored lateral position estimates from the memory tocalculate the first and/or second lateral position value.

Referring still to method 1400, operation 1420 includes calculating thefirst lateral position value using the first number of lateral positionestimates, while operation 1422 includes calculating the second lateralposition value using the second number of lateral position estimates.Depending on the approach, the first and/or second number of lateralposition estimates may be used to calculate the respective first and/orsecond lateral position values differently. For instance, in someapproaches the first and/or second lateral position values may becalculated by averaging the first and/or second number of lateralposition estimates, e.g., according to a number of samples. However, inother approaches the first and/or second lateral position values may becalculated by implementing a weighted averaging, e.g., such that ahigher weight is assigned to the most recent lateral position estimates(samples), and a lower weight is assigned to older estimates. In someapproaches, the way in which the first and/or second lateral positionvalues are calculated may depend on the corresponding number of lateralposition estimates that are available.

Operation 1424 includes using the second lateral position value alongwith the lateral position value to control the tape head actuator.According to some approaches, the first and second lateral positionvalues may be averaged together to form an average lateral positionvalue which may thereby be used to control the tape head actuator, e.g.,in a conventional manner. However, in other approaches the first andsecond lateral position values may be independently used to control thetape head actuator as desired. For example, operation 1424 may includeimplementing the first and/or second lateral position values in one ormore track-following servo control loops, e.g., as shown in FIG. 9.

It should also be noted that in some approaches more than two lateralposition values may be calculated from respective lateral positionestimates, and used to control a tape head actuator. Depending on thedesired approach, a third, fourth, fifth, sixth, etc. lateral positionvalue may be calculated from respective lateral position estimates, andused to control a tape head actuator. According to an illustrativeapproach which is in no way intended to limit the invention, method 1400may further include receiving signals corresponding to windows of athird servo pattern from a third servo channel, a lateral positionestimate for each of a third number of the windows of the third servopattern, the third number of the windows of the third servo patternaccording to the number of the windows of the servo pattern, calculatinga third lateral position value using the lateral position estimates forthe third number of the windows, and using the third lateral positionvalue along with the lateral position value and the second lateralposition value to control the tape head actuator. Moreover, as alludedto above, each of the servo patterns are preferably a respective highdensity servo track on the magnetic tape.

Track-following servo control loops may operate at differentfrequencies, e.g., depending the speed of tape, the number of lateralposition estimates used to calculate a lateral position value, length ofthe windows of the HD servo pattern, etc. Looking to FIG. 15, aflowchart of a method 1500 involving track-following control loops isillustrated in accordance with one embodiment. The method 1500 may beperformed in accordance with the present invention in any of theenvironments depicted in FIGS. 1-11, among others, in variousembodiments. Of course, more or less operations than those specificallydescribed in FIG. 15 may be included in method 1500, as would beunderstood by one of skill in the art upon reading the presentdescriptions.

Each of the steps of the method 1500 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1500 may be partially or entirely performed by acontroller, a processor, etc., or some other device having one or moreprocessors therein. The processor, e.g., processing circuit(s), chip(s),and/or module(s) implemented in hardware and/or software, and preferablyhaving at least one hardware component may be utilized in any device toperform one or more steps of the method 1500. Illustrative processorsinclude, but are not limited to, a central processing unit (CPU), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), etc., combinations thereof, or any other suitablecomputing device known in the art.

As shown in FIG. 15, operation 1502 of method 1500 includes calculatingan operating frequency of a track-following control loop using a currentspeed of a magnetic tape and size (e.g., length) of the window length.According to an exemplary approach, which is in no way intended to limitthe invention, the operating frequency of the track-following controlloop may be calculated using Equation 1 below.

$\begin{matrix}{f = \frac{v_{i}}{\left( {D_{w} \times c_{i}} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Here, ‘f’ represents the operating frequency which may be measured inHz, while “v_(i)” corresponds to the current speed of the magnetic tapewhich may be measured in meters per second. Moreover, “D_(w)”corresponds to the length of the window (measured along a longitudinalaxis of the tape) of a HD servo track on the magnetic tape which may bemeasured in meters, and “c_(i)” corresponds to the number of lateralposition estimates (e.g., the sampling rate) determined to be used tocalculate a lateral position value as described above. Depending on thedesired approach, Equation 1 may be implemented by a controller, aprocessor, etc., or some other device having one or more processorstherein which may be implemented as part of a tape drive.

Referring still to method 1500, operation 1504 includes applying theoperating frequency to the track-following control loop, e.g., as seenin FIG. 9.

It should be noted that although the various embodiments included aboveare described with reference to determining a lateral position value,any one or more of these embodiments may be implemented to form othervalues (estimates) that are desirable in order to perform efficientservo control, e.g., including a velocity value, a magnetic tape skewvalue (relative to an orientation of a magnetic head), etc. Accordingly,any one or more of the embodiments described herein may further includereceiving additional estimates from one or more servo channels. Forinstance, the additional estimates may include magnetic tape velocityestimates derived from a TBS, magnetic tape skew estimates (relative toan orientation of the magnetic head) derived from a TBS, etc., dependingon the desired approach.

It follows that various embodiments described herein introducetechniques which are able to improve the process of determining thelateral position of a tape head by combining (e.g., averaging) lateralposition estimates from windows of a HD servo pattern preferably havinga fixed length, while maintaining a constant spatial frequencyresolution. As a result, various techniques included hereinsignificantly improve the spatial frequency resolution of a HD servochannel, the estimation delay/frequency as well as position estimationresolution/noise. As described above, this may be achieved by usingmultiple lateral position estimates derived from one or more HD servotrack windows to calculate the lateral position of a tape head.Moreover, a servo channel (or firmware) may be used to calculate severallateral position estimates to improve the resolution of the estimate,e.g., by reducing the noise in the estimate. This allows a control loopto operate at a lower rate, while improving PES performance. However,for lower tape speeds, each lateral position estimate may be used tocontrol the position of a tape head relative to a magnetic medium, e.g.,due to the delay associated with low tape speeds.

These improvements are particularly apparent when compared to theconventional limits on PES utilization. Specifically, conventionalimplementations often drop position estimates when faced with high tapespeeds in view of the clock rate of the microprocessor and thecomputation complexity of the track-following controller. Conventionalimplementations also often experience either unmanageable delays orsignificantly inaccurate lateral position approximations of a tape headduring slow tape speeds.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portableCD-ROM, a DVD, a memory stick, a floppy disk, a mechanically encodeddevice such as punch-cards or raised structures in a groove havinginstructions recorded thereon, and any suitable combination of theforegoing. A computer readable storage medium, as used herein, is not tobe construed as being transitory signals per se, such as radio waves orother freely propagating electromagnetic waves, electromagnetic wavespropagating through a waveguide or other transmission media (e.g., lightpulses passing through a fiber-optic cable), or electrical signalstransmitted through a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Moreover, a system according to various embodiments may include aprocessor and logic integrated with and/or executable by the processor,the logic being configured to perform one or more of the process stepsrecited herein. By integrated with, what is meant is that the processorhas logic embedded therewith as hardware logic, such as an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA), etc. By executable by the processor, what is meant is that thelogic is hardware logic; software logic such as firmware, part of anoperating system, part of an application program; etc., or somecombination of hardware and software logic that is accessible by theprocessor and configured to cause the processor to perform somefunctionality upon execution by the processor. Software logic may bestored on local and/or remote memory of any memory type, as known in theart. Any processor known in the art may be used, such as a softwareprocessor module and/or a hardware processor such as an ASIC, a FPGA, acentral processing unit (CPU), an integrated circuit (IC), etc.

It will be clear that the various features of the foregoing systemsand/or methodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer.

The inventive concepts disclosed herein have been presented by way ofexample to illustrate the myriad features thereof in a plurality ofillustrative scenarios, embodiments, and/or implementations. It shouldbe appreciated that the concepts generally disclosed are to beconsidered as modular, and may be implemented in any combination,permutation, or synthesis thereof. In addition, any modification,alteration, or equivalent of the presently disclosed features,functions, and concepts that would be appreciated by a person havingordinary skill in the art upon reading the instant descriptions shouldalso be considered within the scope of this disclosure.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A tape drive-implemented method, comprising:determining a length of a window of a servo pattern to use forcalculating a lateral position estimate; determining a number of thewindows of the servo pattern to use for calculating a lateral positionvalue; calculating a lateral position estimate for each of the number ofthe windows of the servo pattern using signals which correspond to eachof the number of the windows; calculating the lateral position value byusing the lateral position estimates; and using the lateral positionvalue to control a tape head actuator.
 2. The tape drive-implementedmethod as presented in claim 1, comprising: determining a current speedof a magnetic tape, wherein determining the number of the windows of theservo pattern is based on the current speed of the magnetic tape.
 3. Thetape drive-implemented method as presented in claim 1, comprising: usingthe signals which correspond to each of the number of the windows tocalculate a tape head skew estimation value.
 4. The tapedrive-implemented method as presented in claim 1, comprising:calculating a lateral position estimate for each of a second number ofwindows of a second servo pattern, the second number of the windows ofthe second servo pattern according to the number of the windows of theservo pattern; calculating a second lateral position value using thelateral position estimates for the second number of the windows; andusing the second lateral position value along with the lateral positionvalue to control the tape head actuator.
 5. The tape drive-implementedmethod as presented in claim 1, comprising: calculating an operatingfrequency of a track-following control loop using a current speed of amagnetic tape and the length of the window; and applying the operatingfrequency to the track-following control loop.
 6. The tapedrive-implemented method as presented in claim 5, wherein an equation isused to calculate the operating frequency of the track-following controlloop, wherein the equation is presented as:${f = \frac{v_{i}}{\left( {D_{w} \times c_{i}} \right)}},$ wherein “f”represents the operating frequency, wherein “v_(i)” corresponds to thecurrent speed of the magnetic tape, wherein “D_(w)” corresponds to thelength of the window, wherein “c_(i)” corresponds to the number oflateral position estimates.
 7. The tape drive-implemented method aspresented in claim 1, wherein calculating the lateral position valueincludes: calculating an arithmetic mean of the lateral positionestimates.
 8. The tape drive-implemented method as presented in claim 1,wherein calculating the lateral position value includes: calculating aweighted average of the lateral position estimates.
 9. A computerprogram product comprising a computer readable storage medium havingprogram instructions embodied therewith, wherein the computer readablestorage medium is not a transitory signal per se, the programinstructions executable by a processor to cause the processor to:determine, by the processor, a length of a window of a servo pattern touse for calculating a lateral position estimate; determine, by theprocessor, a number of the windows of the servo pattern to use forcalculating a lateral position value; calculate, by the processor, alateral position estimate for each of the number of the windows of theservo pattern using signals which correspond to each of the number ofthe windows; calculate, by the processor, the lateral position value byusing the lateral position estimates; and use, by the processor, thelateral position value to control a tape head actuator.
 10. The computerprogram product as presented in claim 9, the program instructionsexecutable by the processor to cause the processor to: determine, by theprocessor, a current speed of a magnetic tape, wherein determining thenumber of the windows of the servo pattern is based on the current speedof the magnetic tape.
 11. The computer program product as presented inclaim 9, the program instructions executable by the processor to causethe processor to: use, by the processor, the signals which correspond toeach of the number of the windows to calculate a tape head skewestimation value.
 12. The computer program product as presented in claim9, the program instructions executable by the processor to cause theprocessor to: calculate, by the processor, a lateral position estimatefor each of a second number of windows of a second servo pattern, thesecond number of the windows of the second servo pattern according tothe number of the windows of the servo pattern; calculate, by theprocessor, a second lateral position value using the lateral positionestimates for the second number of the windows; and use, by theprocessor, the second lateral position value along with the lateralposition value to control the tape head actuator.
 13. The computerprogram product as presented in claim 9, the program instructionsexecutable by the processor to cause the processor to: calculate, by theprocessor, an operating frequency of a track-following control loopusing a current speed of a magnetic tape and the length of the window;and apply, by the processor, the operating frequency to thetrack-following control loop.
 14. The computer program product aspresented in claim 13, wherein an equation is used to calculate theoperating frequency of the track-following control loop, wherein theequation is presented as:${f = \frac{v_{i}}{\left( {D_{w} \times c_{i}} \right)}},$ wherein “f”represents the operating frequency, wherein “v_(i)” corresponds to thecurrent speed of the magnetic tape, wherein “D_(w)” corresponds to thelength of the window, wherein “c_(i)” corresponds to the number oflateral position estimates.
 15. The computer program product aspresented in claim 9, wherein calculating the lateral position valueincludes: calculating an arithmetic mean of the lateral positionestimates.
 16. The computer program product as presented in claim 9,wherein calculating the lateral position value includes: calculating aweighted average of the lateral position estimates.
 17. A tape drive,comprising: a controller comprising logic integrated with and/orexecutable by the controller to cause the controller to: determine alength of a window of a servo pattern to use for calculating a lateralposition estimate; determine a number of the windows of the servopattern to use for calculating a lateral position value; calculate alateral position estimate for each of the number of the windows of theservo pattern using signals which correspond to each of the number ofthe windows; calculate the lateral position value by using the lateralposition estimates; and use the lateral position value to control a tapehead actuator.
 18. The tape drive as presented in claim 17, comprisinglogic integrated with and/or executable by the controller to cause thecontroller to: use the signals which correspond to each of the number ofthe windows to calculate a tape head skew estimation value.
 19. The tapedrive as presented in claim 17, wherein calculating the lateral positionvalue includes: calculating an arithmetic mean of the lateral positionestimates.
 20. The tape drive as presented in claim 17, whereincalculating the lateral position value includes: calculating a weightedaverage of the lateral position estimates.